Press and magnet manufacturing method

ABSTRACT

A press machine  10  includes a die  12  with a through hole  12   a  that defines a cavity, a first press surface  14   a  and a second press surface  16   a  for pressing a magnetic powder  18  loaded in the cavity, and magnetic field generating means for applying an aligning magnetic field to the magnetic powder  18  in the cavity. At least one of the first and second press surfaces  14   a  and  16   a  has a region made of a material having a Vickers hardness that is higher than 200 but equal to or lower than 450. In pressing the powder under the aligning magnetic field, the press machine  10  minimizes the disturbance in the orientation of the powder.

TECHNICAL FIELD

The present invention relates to a powder press machine and a method forproducing a magnet. More particularly, the present invention relates toa method for producing a rare-earth sintered magnet and a powder pressmachine for producing such a magnet.

BACKGROUND ART

Rare-earth sintered magnets currently used extensively in variousapplications include rare-earth-cobalt based magnets andrare-earth-iron-boron based magnets. Among other things, therare-earth-iron-boron based magnets (which will be referred to herein as“R-T-(M)-B based magnets”, where R is one of the rare-earth elementsincluding Y, T is either Fe alone or a mixture of Fe, Co and/or Ni, M isan additive element (e.g., at least one of Al, Ti, Cu, V, Cr, Ni, Ga,Zr, Nb, No, In, Sn, Hf, Ta and W) and B is either boron alone or amixture of boron and carbon) are used more and more often in variouselectronic appliances. This is because an R-T-(M)-B based magnetexhibits a higher maximum energy product than any of various other typesof magnets and yet is relatively inexpensive.

As the applications of those rare-earth sintered magnets expand, itbecomes increasingly necessary to produce magnets in various shapes. Forexample, to produce a high-performance motor, a number of stronganisotropic magnets with a curved surface are required. To produce suchan anisotropic magnet, a powder compact needs to be compacted into adesired shape by pressing a magnetic powder that has been aligned undera magnetic field. A high-performance rotating machine such as a motoruses a plurality of thin-plate magnets with a C- or arched crosssection. The performance of such a rotating machine cannot be improvedsufficiently just by increasing the magnetic force of the magnets. Inaddition, the resultant magnet shape and magnetic field distribution inthe vicinity of the magnet surface also have to be just as designed.

In the prior art, the pair of punches of a press machine has curvedpress surfaces, thereby obtaining a powder compact with desired curvedsurfaces. The conventional punches may be made of a cemented carbide(e.g., a WC—Ni based alloy) and the press surfaces thereof may be mirrorpolished.

However, the present inventors discovered and confirmed via experimentsthat if the press surfaces were mirror-polished curved surfaces while amagnetic powder was uniaxially pressed under an aligning magnetic field,then the orientation of the magnetic powder was disturbed and theresultant magnet performance was not so good. This problem is quitenoticeable particularly when the pressing direction is substantially thesame as the direction of the aligning magnetic field.

If permanent magnets are made of such a compact with the disturbedorientation and used to produce a motor, then a non-negligible degree ofcogging will be created in the torque of the motor. The “torque cogging”is a torque variation resulting from a variation in the magneticresistance of the magnetic circuit of a motor with the rotationalposition of a rotor. The magnitude of this torque variation is usuallysmall. However, if the cogging torque phenomenon occurs in a powersteering motor, for example, then that variation could be quite sensibleto some drivers. This torque cogging becomes even more perceivable whenthere is such disturbed orientation in the convex portion of each magnet(i.e., a portion of the motor facing a coil).

The above-mentioned problem that a magnetic powder, which has beenaligned under a magnetic field and is now being pressed uniaxially, canhave disturbed orientation arises not only when the press surfaces arecurved but also when one of the two press surface has a region that istilted with respect to the pressing direction. This phenomenon occursduring the manufacturing process of magnets in any of various shapes.

Also, a slicing technique is often used as a method for filling a cavitywith a magnetic powder. For example, as disclosed in Japanese Laid-OpenPublication No. 2000-248301, a feeder box (or a feeder cup) is slid overa cavity, a powder in the feeder box is loaded into the cavity byutilizing the weight of the powder itself, and the upper portion of theloaded magnetic powder is pressed downward by some pressing means suchas an agitator (also called a “shaker”) provided within the feeder box13.

However, the surface of a magnetic powder that has been loaded by such aslicing technique is not always parallel to the surface of a die (i.e.,the bottom of the cavity) but may be either tilted in the direction inwhich the agitator (or the feeder box) moves or even winding. In thatcase, even if the magnetic powder is pressed between two mutuallyparallel press surfaces, at least a portion of the upper press surface(i.e., the surface of the upper punch) contacts with the surface of themagnetic powder obliquely. If the magnetic powder that has been alignedunder the magnetic field is pressed in such a state, then the magneticpowder particles in the vicinity of the press surface will also havedisturbed orientation due to the movement of the compressed magneticpowder just as described above.

DISCLOSURE OF INVENTION

In order to overcome the problems described above, an object of thepresent invention is to provide a press machine to make a magneticpowder compact just as intended, and a method for producing a magnet,with the disturbance in the orientation of the magnetic powderminimized.

A press machine according to the present invention includes: a die witha through hole that defines a cavity; a first press surface and a secondpress surface for pressing a magnetic powder loaded in the cavity; andmagnetic field generating means for applying an aligning magnetic fieldto the magnetic powder in the cavity. At least one of the first andsecond press surfaces has a first region, which is made of a firstmaterial having a Vickers hardness that is higher than 200 but equal toor lower than 450, thereby achieving the object described above.

The at least one press surface may further have a second region, whichis made of a second material having a higher Vickers hardness than thefirst material.

In one preferred embodiment, the at least one press surface has a regionwhich is tilted with respect to a pressing direction.

The first and second materials preferably have permeabilities of 1.01 orless, more preferably 1.001 or less.

The first material is preferably a BeCu alloy.

The BeCu alloy preferably includes 96.9 mass % to 98.2 mass % of Cu and1.6 mass % to 2.0 mass % of Be.

In one preferred embodiment, the at least one press surface is curved.

A method for producing a magnet according to the present inventionincludes the steps of: preparing a magnetic powder; loading the magneticpowder into a cavity; aligning the magnetic powder by applying analigning magnetic field to the magnetic powder in the cavity; anduniaxially pressing the aligned magnetic powder between two opposedpress surfaces to make a compact. At least one of the two press surfaceshas a region that is tilted with respect to the surface of the magneticpowder loaded and/or a pressing direction and also has a first region,which is made of a first material with a Vickers hardness that is higherthan 200 but equal to or lower than 450, thereby achieving the objectdescribed above.

The at least one press surface may further have a second region, whichis made of a second material having a higher Vickers hardness than thefirst material.

The first and second materials preferably have permeabilities of 1.01 orless, more preferably 1.001 or less.

The first material is preferably a BeCu alloy.

The BeCu alloy preferably includes 96.9 mass % to 98.2 mass % of Cu and1.6 mass % to 2.0 mass % of Be.

In one preferred embodiment, the at least one press surface is curved.

The step of uniaxially pressing may be the step of making a compact witha C- or arched cross section.

In another preferred embodiment, the magnetic powder is loaded into thecavity by a slicing technique. The magnetic powder preferably has a meanparticle size (i.e., FSS particle size) of 2 μm to 10 μm.

In still another preferred embodiment, the aligning magnetic field isparallel to the pressing direction. The aligning magnetic fieldpreferably has a strength of 0.5 MA/m to 2.0 MA/m. The aligning magneticfield may be either a static magnetic field or a pulse magnetic field.

In yet another preferred embodiment, the magnetic powder includes arare-earth alloy powder.

A motor according to the present invention includes a magnet produced byone of the methods described above.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1( a) and 1(b) are schematic representations showing the mainportion of a powder press machine 10 according to a preferredembodiment.

FIG. 2 is a perspective view of an arched rare-earth magnet to beproduced as a preferred embodiment of the present invention.

FIG. 3( a) is a cross-sectional view schematically illustrating thestate of a powder during an early stage of a pressing process step beingperformed by a conventional press machine, and FIG. 3( b) is across-sectional view schematically illustrating the state of the powderduring a late stage of the pressing process step.

FIG. 4( a) is a cross-sectional view schematically illustrating thestate of a powder during an early stage of a pressing process step beingperformed by a press machine according to a preferred embodiment, andFIG. 4( b) is a cross-sectional view schematically illustrating thestate of the powder during a late stage of the pressing process step.

FIG. 5 is a perspective view schematically illustrating how the lowerpunch 16 of the press machine according to the preferred embodimentlooks like after the pressing process step.

FIGS. 6( a), 6(b), 6(c) and 6(d) are perspective views schematicallyillustrating lower punches 16 which may be used in various preferredembodiments of the present invention.

FIG. 7( a) is a graph showing the cogging torque of a motor that wasproduced with a magnet representing a specific example of the presentinvention, and FIG. 7( b) is a graph showing the cogging torque of amotor that was produced with a magnet representing a comparativeexample.

BEST MODE FOR CARRYING OUT THE INVENTION

In a press machine according to the present invention, at least one ofthe two press surfaces thereof for pressing magnetic powder particles,which are aligned in a cavity under a magnetic field, has a region(first region) which is made of a material (first material) with aVickers hardness that is higher than 200 but equal to or lower than 450.Having a hardness falling within this range, the first region of thepress surface is deformed plastically under the force pressing themagnetic powder particles, thereby creating fine unevennesscorresponding to the shapes of the magnetic powder particles in thefirst region of the press surface. The fine unevenness that has beenformed in the first region of the press surface functions so as to catchthe magnetic powder particles and prevent them from moving. Accordingly,if a portion of the press surface on which the orientation is oftendisturbed includes the first region, the disturbance in the orientationof the magnetic powder particles in the vicinity of the first region ofthe press surface can be minimized and the powder particles in thecompact can be aligned parallel to the direction of the aligningmagnetic field. In this case, at least one of the two press surfaces mayhave the first region. The region other than the first region (i.e., asecond region) may be made of a material (i.e., a second material) witha higher Vickers hardness than the first material.

The disturbance in the orientation of magnetic powder particles in thevicinity of the press surface also affects even magnetic powderparticles inside of a compact due to magnetic interactions. As a result,the orientation direction of the powder particles inside of the compactbecomes no longer parallel to the direction of the aligning magneticfield and the magnetic properties of the resultant sintered magnetdeteriorate. However, according to the present invention, suchdisturbance in the orientation of magnetic powder particles can beminimized.

Particularly, in a so-called “parallel pressing” process in which thedirection of the aligning magnetic field is the same as the pressingdirection, the magnetic powder particles easily move perpendicularly tothe pressing direction due to the force applied from the press surfaces.The resultant disturbance in orientation seriously affects the magneticproperties of the sintered magnet. Thus, the press machine of thepresent invention can be used particularly effectively in thatsituation.

The magnetic powder particles that have been loaded into the cavity aremovable especially easily under the force applied from the presssurfaces when the press surfaces have a region that is tilted withrespect to the pressing direction (e.g., when the press surfacesthemselves are tilted with respect to the pressing direction) or whenthe press surfaces are curved. It should be noted that the phrase “thepress surfaces are tilted with respect to the pressing direction” meansthat the press surfaces are not perpendicular to the pressing direction.

Particularly when a compact having a C- or arched cross section (e.g., asintered magnet for a motor) is made, the two press surfaces are bothcurved and have mutually different shapes (i.e., their cross-sectionalshapes as taken in the pressing direction). Accordingly, the orientationis often disturbed on both of the two press surfaces. In that case, bothof the two press surfaces are preferably made of a material with aVickers hardness falling within the range described above.

Even if the two press surfaces are mutually parallel flat planes, thesurfaces of the magnetic powder that has been loaded into the cavity maynot be flat. In that case, the press surfaces in contact with thesurfaces of the magnetic powder are also tilted. Even so, by using presssurfaces that are made of a material with a Vickers hardness fallingwithin the range specified above, the disturbance in the orientation ofthe magnetic powder particles can also be minimized. Accordingly, theeffects of the present invention are achieved when the cavity is filledwith the magnetic powder material by a mass-producible slicingtechnique, for example.

It should be noted that if the material of the press surfaces had aVickers hardness exceeding 450, then the press surfaces could not beplastically deformed well enough to achieve the effects of minimizingthe orientation disturbance fully. However, if the Vickers hardness were200 or less, then the shape of the resultant compact would be somewhatdifferent from the predetermined shape. Accordingly, it might take alonger time to carry out the subsequent process step of reshaping thecompact or the life of the press surfaces might be shorter thanexpected. Considering the shape precision of the compact and the life ofthe punches (or press surfaces), a material with a Vickers hardness ofat least 250 is more preferably used.

If the press surfaces are made of a material with a permeability of 1.01or less, then the aligning magnetic field is not affected by thismaterial at all. Thus, an aligning magnetic field with a predetermineddirection can be applied to the magnet powder within the cavity. Morepreferably, the material of the press surfaces has a permeability of1.001 or less. In contrast, if the press surfaces are made of a Cr—Nibased stainless steel (SUS 304 according to JIS standards and having apermeability of about 1.02 to about 1.06), for example, then the fluxdensity will be relatively high around the center of the cavity, themagnet powder will be concentrated toward that region, and theorientation will be disturbed easily. A BeCu alloy (among other things,a BeCu alloy including 96.9 mass % to 98.2 mass % of Cu and 1.6 mass %to 2.0 mass % of Be) is preferably used.

A BeCu alloy is also less expensive than a cemented carbide. Also, sincethe press surfaces are deformed plastically as a result of the pressingprocess, there is no need to mirror-polish those surfaces. Accordingly,a material that has just been shaped by an electrical discharge, forexample, may be used as it is, thus further cutting down the costadvantageously. Furthermore, the BeCu alloy is also machinable easilyand can be used effectively as a material for the punches for producingnumerous sorts of magnets in small quantities. It should be noted thateven when the BeCu alloy is used, the pressing process could be carriedout repeatedly up to about 1,000 shots, for example.

Hereinafter, preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings. In the followingillustrative preferred embodiments, the two press surfaces are bothsupposed to be curved. However, the present invention is in no waylimited to those specific preferred embodiments.

Press Machine

FIGS. 1( a) and 1(b) illustrate the main portion of a powder pressmachine 10 according to this preferred embodiment. The press machine 10shown in FIG. 1 includes a die 12 with a through hole (i.e., die hole)12 a to define a cavity and an upper punch 14 and a lower punch 16 forcompressing a magnetic powder 18 within the through hole. The end of theupper punch 14, including its compression axis, has a convexcross-sectional shape, while the end of the lower punch 16 has a concavecross-sectional shape. A die set, consisting of the die 12, upper punch14 and lower punch 16, is connected to a driver (not shown) so as toperform a vertical motion as required for a pressing process. The pressmachine 10 of this preferred embodiment operates in basically the sameway as any known press machine.

The shape of the die set for use in this preferred embodiment isdesigned so as to produce an arched thin-plate rare-earth magnet 20 asshown in FIG. 2. This rare-earth magnet 20 is magnetized parallel to thearrow A shown in FIG. 2 (i.e., in the pressing direction). Therare-earth magnet 20 shown in FIG. 2 may be used as a part of a motor orany other rotating machine. When the magnet 20 is used for a motor, theshape of the magnet 20 is preferably designed so as to produce a skewand thereby reduce cogging torque.

Look at FIG. 1( a) again.

The cavity is formed over the upper portion of the lower punch 16 thathas been partially inserted into the through hole 12 a of the die 12,and is filled with the magnetic powder 18. The cavity is a compactingspace to be defined by the press surface 16 a of the lower punch 16 andthe inner surface of the through hole 12 a.

The powder is loaded into the cavity by transporting a feeder box (notshown), which is filled with the magnetic powder, to over the cavity anddropping the powder into the cavity through the bottom (opening) of thefeeder box. The cavity cannot be filled with the powder uniformly justby letting the powder drop due to its gravitation. Accordingly, themagnetic powder 18 is preferably stuffed into the cavity by driving ashaker (not shown), provided inside of the feeder box, horizontally (seeJapanese Laid-Open Publication No. 2000-248301 mentioned above).

While the feeder box is retreating from over the cavity, the top of themagnetic powder 18 is sliced off with the bottom edge of the feeder box,thereby loading a predetermined amount of powder 18 to be compacted intothe cavity highly precisely. In the meantime, the surface of themagnetic powder 18 loaded may be partially tilted with respect to thesurface of the die 12.

This press machine 10 is characterized in that the press surfaces 14 aand 16 a of the upper and lower punches 14 and 16 are made of a BeCualloy (e.g., BC-2 produced by Sumitomo Special Metals Co., Ltd). TheBeCu alloy is a non-magnetic material with a Vickers hardness higherthan 200 but equal to or lower than 450 and a permeability of 1.001 orless. A BeCu alloy including 96.9 mass % to 98.2 mass % of Cu and 1.6mass % to 2.0 mass % of Be satisfies these Vickers hardness andpermeability ranges.

In the press machine 10 of this preferred embodiment, the press surfaces14 a and 16 a are fully made of the BeCu alloy. Accordingly, thedisturbance in the orientation of the magnetic powder 18 can beminimized effectively as will be described later. In this example, thepress surfaces 14 a and 16 a are completely made of the BeCu alloy.However, the effects of reducing the disturbance in the orientation ofthe magnetic powder 18 are achievable if at least one of the two presssurfaces 14 a and 16 a has a region that is made of the BeCu alloy(i.e., a first region).

It should be noted that even when the press surfaces 14 a and 16 a arefully made of the BeCu alloy, just the press surfaces 14 a and 16 a, notthe whole upper and lower punches 14 and 16, need to be made of the BeCualloy. Nevertheless, to achieve a sufficient degree of plasticdeformation, portions with a thickness of about 2 mm or more, includingthe press surfaces, are preferably made of the BeCu alloy. Normally, ifthe press surfaces having predetermined shapes are made of a BeCu alloyblock, these conditions are satisfied easily. Since the BeCu alloy hassome elasticity, a clearance of about 5 μm to about 60 μm is preferablyprovided between the side surfaces of the upper punch 14 and throughhole 12 a and/or between the side surfaces of the lower punch 16 andthrough hole 12 a.

When the cavity has been filled with the magnetic powder 18, the upperpunch 14 starts to be lowered. The press surface 14 a of the upper punch14 presses the upper surface of the underlying powder 18 as shown inFIG. 1( b). After the magnetic powder 18 has been completely enclosed bythe upper punch 14, lower punch 16 and die 12, a magnetic fieldgenerating coil (not shown) applies an aligning magnetic field to thepowder 18 in the cavity. The magnetic flux is lead into the upper andlower punches 14 and 16, thereby making the direction of the aligningmagnetic field within the cavity identical with the pressing direction(i.e., the direction in which the upper punch operates). Under thisaligning magnetic field, the powder particles being pressed are alignedwith the direction of the magnetic field. The aligning magnetic fieldpreferably has a strength of 0.5 MA/m to 2.0 MA/m. The aligning magneticfield may be either a static magnetic field or a pulse magnetic field.

With the aligning magnetic field applied to the powder, the magneticpowder within the cavity is compressed and compacted by the upper andlower punches 14 and 16, thereby making a powder compact 24. In thecourse of this pressing process, the particles of the magnetic powder 18being pressed receive different stresses (or pressures) depending ontheir locations. After the compact 24 has been obtained, the upper punch14 will be raised and the lower punch 16 will push the compact 24upward, thereby extracting the compact 24 from the die hole 12 a.

FIG. 3( a) schematically illustrates how the magnetic powder particles18 a are oriented in the initial stage of a pressing process performedby a conventional press machine. On the other hand, FIG. 3( b)schematically illustrates how the magnetic powder particles 18 a areoriented in the late stage of the pressing process.

The respective magnetic powder particles 18 a under the aligningmagnetic field are not only oriented toward the direction of thealigning magnetic field but also are strongly coupled togethermagnetically. As a result, the powder particles 18 a are aligned withthe direction of the aligning magnetic field as shown in FIG. 3( a). Asthe distance between the upper and lower punches 14′ and 16′ isdecreased with the aligning magnetic field applied, non-uniformpressures (or stresses) are applied to the respective portions of thepowder being pressed because the punch surfaces 14 a′ and 16 a′ arecurved. However, if the press surfaces 14 a′ and 16 a′ are made of amuch harder cemented carbide and are mirror-polished smooth surfaces,then the powder particles will slide on the smooth press surfaces 14 a′and 16 a′ laterally (i.e., directions including a componentperpendicular to the pressing direction). As a result, the orientationis disturbed as shown in FIG. 3( b).

In contrast, according to this preferred embodiment, such sliding of thepowder particles in the vicinity of the press surfaces is reducedsignificantly, thereby minimizing the disturbance of the orientation asshown in FIGS. 4( a) and 4(b). The reason is as follows. The presssurfaces 14 a and 16 a (made of the BeCu alloy) of the upper and lowerpunches 14 and 16 are deformed plastically under the forces receivedfrom the powder particles 18 a. As a result, as schematically shown inFIG. 5, fine unevenness is created on the BeCu alloy press surface 16 aof the lower punch 16. This unevenness is represented by a surfaceroughness Ra of 0.05 μm to 12.5 μm. The press surface 16 a isillustrated in FIG. 5. However, a similar unevenness is created on thepress surface 14 a, too. Such fine unevenness created on the presssurfaces 14 a and 16 a reduce the sliding of the powder particles 18 asignificantly, thereby minimizing the disturbance of the orientation.

As described above, the magnetic powder particles 18 a are magneticallycoupled together under the magnetic field. Accordingly, the motion ofthe powder particles inside of the cavity is dominated by that of thepowder particles in the vicinity of the press surfaces 14 a and 16 a.For that reason, just by deforming the press surfaces 14 a and 16 aplastically in accordance with the shapes of the powder particles 18 acontacting with the press surfaces 14 a and 16 a, the decrease in thedegree of alignment can be minimized for all of the powder particles 18a within the cavity.

It should be noted that the unevenness of the press surfaces 14 a and 16a is transferred onto the surface of the resultant compact 24. However,by polishing the surface of the magnet after that depending on thenecessity, such transferred patterns can be removed easily and themagnet surface can be smoothed out.

In the preferred embodiment described above, the press surfaces 14 a and16 a are fully made of the BeCu alloy. Alternatively, as shown in FIGS.6( a) through 6(d), some region(s) (i.e., first region(s)) 16 b of thepress surface 16 a may be made of the BeCu alloy, while the otherregion(s) (i.e., second region(s)) 16 c may be made of a non-magneticcemented carbide (e.g., WC—Ni with a Vickers hardness of 1,200 and apermeability of 1.0003) or a high manganese steel (with a Vickershardness of 460 and a permeability of 1.004). Naturally, the presssurface 14 a, as well as the press surface 16 a, may also include thefirst and second regions.

The Vickers hardness of the material of the second region 16 c may behigher than that of the material of the first region, may exceed 450,and is preferably 700 or more. If a material with a Vickers hardness of700 or more is used, then the material is hardly worn out even withcontinuous use and may be used over and over again by reforming thefirst region only. As described above, the press surface 16 a ispreferably made of material with a permeability of 1.01 or less.Accordingly, the second region 16 c is also preferably made of amaterial with a permeability of 1.01 or less. The permeability is morepreferably 1.001 or less.

On the press surface 16 a, the first and second regions 16 b and 16 cmay be arranged in various patterns including those illustrated in FIGS.6( a) through 6(d), for example. Specifically, as shown in FIG. 6( a), aplurality of first regions 16 b may be uniformly arranged as smallislands on the front of the press surface 16 a. Alternatively, as shownin FIGS. 6( b) through 6(d), the first region(s) 16 b may be eitherselectively provided or concentrated where the orientation of themagnetic particles is easily disturbed. If a plurality of first regions16 b are provided, the size and shape of each first region 16 b are notparticularly limited but may be appropriately determined according tothe method of forming the press surface 16 a.

The region where the orientation of the magnetic particles is easilydisturbed shifts according to the shape of the press surface 16 a(and/or the press surface 14 a), the relationship between the directionof the aligning magnetic field and the pressing direction, and thedirection in which an agitator moves when the powder is loaded by theslicing technique. Accordingly, the location(s) of the first region(s)16 b may be determined in view of these factors. In the illustratedparallel pressing process, the orientation is disturbed particularlyeasily in the peripheral regions of the press surface 16 a with largetilt angles. Accordingly, in the examples illustrated in FIGS. 6( b)through 6(d), the first regions 16 b are provided in such regions of thepress surface 16 a. In this case, the tilt angle of the press surface 16a is defined with a plane perpendicular to the pressing direction (whichis normally a horizontal plane) used as a reference plane.

The first region(s) 16 b may be provided on the press surface 16 a bythe following method, for example. Specifically, the body portion of thelower punch 16 may be formed with the material of the second region(s)16 c (e.g., a non-magnetic cemented carbide). Then, holes or grooves maybe provided in the regions to be the first regions 16 b and then theBeCu alloy may be injected into, or welded with, those holes or grooves.To minimize the disturbance in the orientation of the powder particles,the BeCu alloy making the first regions 16 b may have a thickness of atleast several μm. To achieve a sufficient degree of plastic deformation,the thickness of the BeCu alloy is preferably 2 mm or more. Thethickness of the BeCu alloy may be appropriately determined by thespecific method of forming the first regions 16 b, for example.

In forming a C- or arched compact for use in a motor, at least a portionof the press surface 16 a that will make the convex surface of thecompact is preferably made of the BeCu alloy. Then, the orientationdisturbance can be minimized and the cogging torque can be reducedeffectively.

Method of Making Alloy Powder

The present invention can be used effectively to produce a rare-earthsintered magnet and is particularly effective in producing an R-T-(M)-Bbased high-performance rare-earth sintered magnet.

Cast flakes of an R-T-(M)-B based rare-earth magnet alloy are preparedby a known strip-casting process. Specifically, an alloy, having acomposition consisting of 30 wt % of Nd, 1.0 wt % of B, 1.2 wt % of Dy,0.2 wt % of Al, 0.9 wt % of Co and Fe and inevitable impurities as thebalance, is melted by a high-frequency melting process, therebyobtaining a molten alloy. The molten alloy is maintained at 1,350° C.and then rapidly cooled by a single roller process to obtain alloy castflakes with a thickness of 0.3 mm. In this case, the conditions of therapid cooling process include a roller peripheral velocity of about 1m/s, a cooling rate of 500° C./min and a supercooling rate of 180° C.The rapid cooling rate is defined at 10²° C./s to 10⁴° C./s.

The rapidly solidified alloy obtained in this manner has a thickness of0.03 mm to 10 mm. The alloy includes R₂T₁₄B crystal grains with aminor-axis size of 0.1 μm to 100 μm and a major-axis size of 5 μm to 500μm and R-rich phases dispersed on the grain boundary of the R₂T₁₄Bcrystal grains. The thickness of the R-rich phases is 10 μm or less. Amethod of making a material alloy by the strip-casting process isdisclosed in U.S. Pat. No. 5,383,978, for example. The R-T-(M)-B basedrare-earth magnet alloy powder has an elongated shape with a high aspectratio and exhibits poor flowability (or compactibility) in thecompressing process.

Next, the coarsely pulverized material alloy is loaded into materialpacks, which are subsequently put on a rack. Thereafter, the rack loadedwith the material packs is transported to the front of a hydrogenfurnace using a material transporter and then introduced into thehydrogen furnace. Then, a hydrogen pulverization process is started inthe hydrogen furnace. That is to say, the material alloy is heated andsubjected to the hydrogen pulverization process inside of the hydrogenfurnace. The material alloy, coarsely pulverized in this manner, ispreferably unloaded after the temperature of the alloy has decreasedapproximately to room temperature. However, even if the material alloyis unloaded while the temperature of the alloy is still high (e.g., inthe range of 40° C. to 80° C.), the alloy is not oxidized so seriouslyunless the alloy is exposed to the air. As a result of this hydrogenpulverization process, the rare-earth alloy is pulverized to a size ofabout 0.1 mm to about 1.0 mm. As described above, before subjected tothis hydrogen pulverization process, the material alloy has preferablybeen pulverized more coarsely into flakes with a mean particle size of 1mm to 10 mm.

After the material alloy has been pulverized by this hydrogenpulverization process, the alloy with increased brittleness ispreferably pulverized more finely and cooled using a cooling machinesuch as a rotary cooler, for example. If the material unloaded still hasa relatively high temperature, then the material should be cooled for arather long time by the rotary cooler.

Thereafter, the material powder, which has been cooled to the vicinityof room temperature by the rotary cooler, is further pulverized evenmore finely by a pulverizer such as a jet mill to make a fine materialpowder. In view of the resultant magnetic properties, the fine powderpreferably has a mean particle size (i.e., FSSS particle size) of 2 μmto 10 μm. In this preferred embodiment, the material powder is finelypulverized using a jet mill within a nitrogen gas atmosphere, therebyobtaining an alloy powder with a mean particle size of about 3.5 μm. Theconcentration of oxygen in this nitrogen gas atmosphere is preferably aslow as about 10,000 ppm. Such a jet mill is disclosed in Japanese PatentGazette for Opposition No. 6-6728, for example. More specifically, theweight of oxygen included in the finely pulverized alloy powder ispreferably adjusted to 6,000 ppm or less by controlling theconcentration of an oxidizing gas (i.e., oxygen or water vapor) in theatmospheric gas for use in the fine pulverization process. This isbecause if the weight of oxygen included in the rare-earth alloy powderexceeds 6,000 ppm, then the total percentage of non-magnetic oxides tothe resultant sintered magnet is too high to achieve good magneticproperties.

Subsequently, 0.3 wt % of lubricant is added to, and mixed with, thisalloy powder in a rocking mixer, thereby coating the surface of thealloy powder particles with the lubricant. As the lubricant, a fattyester diluted with a petroleum solvent may be used. In this preferredembodiment, methyl caproate is used as the fatty ester and isoparaffinis used as the petroleum solvent. Methyl caproate and isoparaffin may bemixed at a weight ratio of 1:9, for example. Such a liquid lubricant notonly prevents the oxidation of the powder particles by coating thesurface thereof but also improves the degree of alignment of the powderbeing pressed and the powder compactibility (i.e., how easy theresultant compact can be removed).

It should be noted that the lubricant is not limited to the exemplifiedtype. For example, methyl caproate as the fatty ester may be replacedwith methyl caprylate, methyl laurylate or methyl laurate. Examples ofpreferred solvents include petroleum solvents such as isoparaffin andnaphthene solvents. The lubricant may be added at any time: before,while or after the fine pulverization process. A solid (dry) lubricantsuch as zinc stearate may also be used instead of, or in addition to,the liquid lubricant.

It should be noted that the powder made by this method has a sharpparticle size distribution and therefore exhibits poor flowability.Accordingly, if the powder is compressed in the same direction as itsorientation direction, then the orientation tends to be disturbed easilyduring the pressing process. Also, by adding a lubricant such as a fattyester, the respective powder particles can be aligned more easily butits flowability deteriorates instead. As a result, the orientation iseasily disturbed due to the pressing. Furthermore, if a high aligningmagnetic field of 0.8 T to 2.0 T, for example, is applied to achievehigh magnetic properties, such orientation disturbance becomes even moresignificant. In that case, significant effects are achieved according tothis preferred embodiment by using a press machine with press surfacesmade of the BeCu alloy.

Method for Producing Rare-Earth Magnet

First, the magnetic powder that has been prepared by the methoddescribed above is compacted by the press machine shown in FIG. 1 underan aligning magnetic field. After the power has been pressed andcompacted in this manner, the resultant powder compact is pushed upwardby the lower punch 16 and removed from this press machine. At this pointin time, patterns reflecting the fine unevenness on the press surfaces14 a and 16 a have been transferred during the pressing process onto thesurfaces of the compact (that have been in contact with the presssurfaces 14 a and 16 a). According to this preferred embodiment, apowder compact with hardly disturbed orientation can be obtained asshown in FIG. 4( b).

To release the compact from the die more easily and more smoothly, thepress surfaces may be coated with a release agent either entirely orpartially before the powder is loaded thereto. The release agent ispreferably obtained by diluting a fatty ester with a solvent.Specifically, examples of preferred fatty esters include methylcaproate, methyl caprylate, methyl laurylate and methyl laurate. Apetroleum solvent such as isoparaffin may be used as the solvent. Andthe fatty ester and solvent may be mixed at a weight ratio of 1:20 to1:1. Optionally, the fatty acid may include 1.0 wt % or less ofarachidic acid.

Next, the compacts are placed onto a sintering plate (with a thicknessof 0.5 mm to 3 mm). The plate may be made of a molybdenum material, forexample. The compacts 24 on the plate are loaded into a sintering case.The sintering case loaded with the compacts 24 is transported into, andsubjected to a known sintering process at, a sintering furnace. As aresult of this sintering process, the compacts turn into sinteredbodies.

Thereafter, the surface of the sintered bodies is polished if necessary.On the surface of the as-sintered compacts, surface patternscorresponding to the fine unevenness on the press surfaces 14 a and 16 aare still left. Part or all of these surface patterns may be removed bythe polishing process. After or instead of this polishing process, theprocess of coating the surface of the sintered bodies with a resin film,for example, may also be carried out. In this manner, final products, orrare-earth magnets, are obtained.

Preferred embodiments of the present invention have been described asbeing applied to a rare-earth magnet having such a shape as being usableeffectively in a motor or any other rotating machine. However, thepresent invention is in no way limited to those specific preferredembodiments.

In the magnet shown in FIG. 2, the upper and lower surfaces thereof areboth curved. However, the effects of the present invention are alsoachieved fully even if just one of the two surfaces is curved. In thatcase, the press surface of the punch to form the non-curved flat surfacemay be made of the same material as the conventional one (e.g., acemented carbide).

Also, as already described with reference to FIG. 1( a), the surface ofthe magnetic powder 18 that has been loaded into the cavity by theslicing filling technique may not be parallel to the surface of the die12 but tilted toward a certain direction or winding. In that case, evenif the two press surfaces are mutually parallel flat planes, those presssurfaces in contact with the surfaces of the magnetic powder will betilted thereto. Accordingly, if the conventional press surfaces,obtained by mirror-polishing a cemented carbide, for example, are used,the orientation disturbance problem arises. However, by using a pressmachine with press surfaces made of the BeCu alloy, the disturbance inthe orientation of the magnetic powder particles can be minimizedthrough the same mechanism as that described above.

EXAMPLE AND COMPARATIVE EXAMPLE

A rare-earth alloy powder was pressed and compacted with the pressmachine 10 including the press surfaces 14 a and 16 a made of a BeCualloy (e.g., BC-2 produced by Sumitomo Special Metals Co., Ltd.) asshown in FIG. 1. In this example, a compact was made so as to have alength of 40 mm as measured in the direction indicated by the arrow B inFIG. 2, a thickness of 7 mm at the center portion thereof as measured inthe direction indicated by the arrow A, and a width of 35 mm as measuredin the direction perpendicular to both of the arrows A and B and to havea compact density of 4.30 g/cm³. The aligning magnetic field of about 1MA/m was applied in the pressing direction (i.e., along the arrow A).Thereafter, the compact was sintered at 1,050° C. for two hours withinan argon atmosphere, thereby making a magnet. This magnet was magnetizedand then the flux density distribution in the vicinity of the surface ofthe magnet was obtained.

For the purpose of comparison, a similar pressing process was carriedout with a press machine including press surfaces made of theconventional cemented carbide, thereby making a magnet as a comparativeexample.

The flux density distribution obtained for this example was better thanthat of the comparative example and showed no abnormal distributionresulting from the decrease in the degree of alignment.

The cogging torque of a motor that was produced using the magnets ofthis example was measured. The results are shown in FIG. 7( a). For thepurpose of comparison, the cogging torque of a motor including themagnets of the comparative example was also measured. The results areshown in FIG. 7( b).

As is clear from the results shown in FIGS. 7( a) and 7(b), the coggingtorque of the motor of this example was sufficiently smaller than thatof the motor of the comparative example. The reason why the coggingtorque of a motor is reduced by the present invention is that theorientation is unlikely to be disturbed in the compact under thepressing process.

INDUSTRIAL APPLICABILITY

In the press machine of the present invention, at least a portion of thepress surface thereof is made of a material with an adequate Vickershardness. Accordingly, while a powder is being pressed under an aligningmagnetic field, it is possible to prevent the powder particles fromsliding on the press surface, thereby minimizing the disturbance in theorientation of the powder.

A powder compact obtained by using such a press machine achieves uniformalignment, and a rare-earth magnet made of such a compact exhibitsexcellent magnetic properties.

If a motor is constructed using magnets produced by the method of thepresent invention, the cogging torque can be reduced.

1. A method for producing a magnet, the method comprising the steps of:preparing a magnetic powder; loading the magnetic powder into a cavity;aligning the magnetic powder by applying an aligning magnetic field tothe magnetic powder in the cavity; and uniaxially pressing the alignedmagnetic powder between two opposed press surfaces to make a compact, atleast one of the two press surfaces having a region that is tilted withrespect to the surface of the magnetic powder loaded and/or a pressingdirection and also having a first region, which is made of a firstmaterial with a Vickers hardness that is higher than 200 but equal to orlower than
 450. 2. The magnet producing method of claim 1, wherein theat least one press surface further has a second region, which is made ofa second material having a higher Vickers hardness than the firstmaterial.
 3. The magnet producing method of claim 2, wherein the firstand second materials have permeabilities of 1.01 or less.
 4. The magnetproducing method of claim 3, wherein the first material is a BeCu alloy.5. The magnet producing method of claim 4, wherein the BeCu alloyincludes 96.9 mass % to 98.2 mass % of Cu and 1.6 mass % to 2.0 mass %of Be.
 6. The magnet producing method of claim 1, wherein the at leastone press surface is curved.
 7. The magnet producing method of claim 6,wherein the step of uniaxially pressing is the step of making a compactwith a C- or arched cross section.
 8. The magnet producing method ofclaim 1, wherein the magnetic powder is loaded into the cavity by aslicing technique.
 9. The magnet producing method of claim 1, whereinthe aligning magnetic field is parallel to the pressing direction. 10.The magnet producing method of claim 1, wherein the magnetic powderincludes a rare-earth alloy powder.